Rotifers are tiny aquatic creatures with options: They can have sex, or they can simply clone themselves. And this ability to go both ways has made them interesting subjects in the study of the evolution of sex.

Now scientists find when these animals are moved between varied environments, they became more likely to seek out a mate, rather than creating the next generation on their own, sans sex.

Why do scientists care? Because sex is messy and costly and is not the easiest way for organisms to propagate, so scientists have long wondered why it evolved at all from the original method — asexual reproduction, aka cloning.

This study indicates that certain environmental conditions allow for the evolution of a higher rate of sex, in spite of the costs it entails, according to the researchers. [237 Reasons We Have Sex]

The drawbacks to sex
Sex between parents shuffles their genes before passing them on — not the simplest way to create offspring.

"It would be so much easier to clone yourself. You don't have to find a mate, you don't have to spend time mating," explained study researcher Lutz Becks, an evolutionary biologist at the University of Toronto.

The list goes on: Sex also has the potential to break up advantageous genetic combinations and, assuming each individual produces the same number of offspring, it produces half as many individuals by the third generation as compared with asexual reproduction.

Theories explaining the evolution of sex focus on its role in shuffling genes as a means of purging harmful mutations or by allowing host species to evolve new defenses to fight off parasites. There is also theoretical evidence that sex evolves more easily in a spatially varied environment, which the researchers tested.

A complicated life cycle
Becks and University of Toronto colleague Aneil Agrawal gave the rotifers either high- or low-quality algae to eat. They then moved some of the rotifers between the high- and low-quality food environments. This "migration" created varied environments for them.

Rotifers have complicated sex lives. If conditions are right, they can switch from asexual reproduction to sexual reproduction, but the change requires two generations — one generation for asexual females to switch from laying cloned eggs to laying eggs that produce sexual females. During the second generation, those sexual females then produce males, and the males and females can mate, producing eggs. Rotifers produce about one offspring per day.

To study how much sex was going on among the rotifers, the scientist looked at the proportion of clones that switched to sexual reproduction and the percentage of sexually produced eggs.

After 13 weeks, they found that sex had declined rapidly among the rotifers living in the homogenous environments of either all high- or all low-quality algae. However, among the rotifers that had to adjust to different environments as the scientists moved them around, the decline in the rate of sex was negligible.

To get a better idea of what was going on, the scientists restarted the experiment, mixing all of the animals together. Because sex among the animals in the homogenous environments had declined markedly, this brought the overall rate of sex down below where it had been 13 weeks prior. However, after the experiment was restarted, the rate of sex among the animals in the varied environments climbed toward its previous rate, leading the scientists to believe that the rate of sex was reaching an equilibrium determined by the environment.

Emperor penguins endure some the harshest conditions on the planet — the Antarctic winter — to satisfy their primal urge for sex. The annual ritual begins with a days-long, up to 75-mile slog to their inshore breeding grounds. Once there, an elaborate courtship of calls and poses reunites old mates and enables young lovers to form lasting bonds.

Copulation itself occurs under the cover of the dark of the polar night. Then the true sign of their devotion begins: Males huddle together to incubate the eggs as the females waddle back out to sea to feast, fattening up to provide for their newly hatched young. Once she returns, males depart in the first of a tradeoff that may, eventually, allow for healthy offspring.

Click on the "Next" label for nine more peeps at sex in the wild.

- John Roach, msnbc.com contributor

Captive pandas require help to get it on

Wichai Taprieu
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AP file

In the wild, biologists say pandas get it on without too much of a hitch. But since the wild population in China's mountain forests hovers around just 1,600, there's a push to boost the panda population in zoos around the world. And that's where the problems lie. Many pandas lack interest in their arranged mates and some inexperienced males who give it the old college try fail to engage the proper body part. As an aid, some zoo keepers attempt to arouse the bears and teach them appropriate technique with specially made DVDs, a.k.a. panda porn. When that fails — and it often does — artificial insemination is considered a last resort. These tricks combined with a better understanding of what makes the panda libido tick are beginning to pay off. The captive population has boomed in recent years.

Male macaques groom females in exchange for sex, according to a study that examined the market underpinnings of monkey sex. According to the research, based on 20-months of observation in an Indonesian nature reserve, a female is three times more likely to mate with a male if he grooms her first. Supply and demand also comes into play: Males spend more time grooming when competition for female attention is greater. Scientists refer to this practice as a biological market.

Male antelopes play hard to get

Jakob Bro-Jorgensen / Zoological Society of London

For highly-desired male topi antelopes in Africa, the frenzied six-week-long mating season is exhausting. Any given female is receptive for about a day, thus she wants to mate as many times as possible, especially with the fittest males. This creates intense competition for high quality antelope sperm, allowing those that possess it to be picky. A study of the phenomenon found the choosy males deliberately select the least mated females and fend off aggressive females they've already mated with. The aggressive female in the center of this image is attacking the male on the left as another female eyes the scene.

Sumatran rhinos tussle before they tango

Tom Uhlman
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AP

Mating for the typically solitary and territorial Sumatran rhinoceros is a drawn-out affair. Studies of the critically endangered species in captivity show that when a female becomes receptive to a male's approach, she'll exhibit increased urine spraying, tail raising and swinging, and vocalizations. Foreplay includes head and genital butting, which can be a bit too much when a female isn't quite ready to tango with a young and aggressive male. But when the tussle is successful, a male will mount, often riding his mate for up to an hour. The image here shows the results of a successful mating at the Cincinnati Zoo. Such captive breeding efforts are a silver lining for conservationists hoping to keep the Sumatran rhino alive — just 300 are thought to remain in the wild.

With a cacophony of clicks, clacks, whistles, and bellows, male walruses swim around the chilly Arctic waters vying for the attention of ice-bound females. Males will also fight off other males that get too close, sending the loser packing. Once the fussing and fighting is done, female harems surround the burliest males in the water for an underwater romp. Though scientists know little about what actually happens under the cover of the waves, they do know walruses are endowed with the mammal world's largest penis bone, called a baculum, which extends up to 30 inches.

Dolphin mating is brief, but bountiful

Noaa
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NOAA

For playful dolphins, mating seems just like another carefree and pleasurable way to pass away the day. Along with some typical male posturing for access to females, there's plenty of chasing, rubbing, nuzzling and stroking that constitutes as foreplay. The belly-to-belly copulation act itself lasts less than a minute, though is often repeated several times over the course of an hour.

Virgin female spiders risk all for a big mate

For some male spiders, sex is the ultimate sacrifice: females eat them as part of the reproductive ritual. But among the East African blood-gorging jumping spider Evarcah culicivora, shown here, males possess the coital-infused cannibalistic urge. Nevertheless, female virgins opt to be deflowered by bigger males before settling in with a small guy for the long haul. Scientists suspect females gamble with their fate once in hopes of producing larger, fitter offspring, but decide not to double down after the flirtation with danger.

As this picture shows, Japanese sap beetles come in big, medium, and small sizes. Generally, in the beetle world, mating success is only bestowed on males with the biggest bodies because they can beat out the competition with brute force. Big sap beetles successfully employ this strategy, but when they do, the medium guys take to the air with their extra long wings and survey for sites where big males are absent. But the little guys have the biggest testicles. This allows them to hang with the big guys and sneak sex behind their backs. Since the little beetles have such big testes, they produce more competitive sperm, upping their chances at siring offspring when they get a shot.

Cycads have 'hot' plant sex

Irene Terry
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Univ. of Utah

Hot sex has ancient roots. The males in a group of plants called cycads, which have been around for at least 250 million years, get all hot and bothered as a means to compel pollen-covered insects deep in their cones to flee forth and find a female to pollinate, according to scientists. The plants begin the process by emitting a fragrance that lures little insects called thrips into their cones. After a few hours of the thrips feasting and rolling around in there, the cycads heat up as much as 25 degrees Fahrenheit, which turns their sweet smelling fragrance into a stench. The thrips flee, some landing on benign-smelling female cycads and thus completing the pollination cycle.